This invention relates in general to methods of potting structures in thermosetting resin compositions and, more specifically to potting structures using such resins including a quantity of a titanate coupling agent to reduce voids.
Titanate coupling agents are well known and ar used for a wide variety of purposes. Primarily, these coupling agents are used to improve the surface bond between an inorganic filler material and an organic polymer matrix by acting as a molecular bridge at the interface between the two materials. The coupling agents, when incorporated into filled polymer systems, have in the past been used to improve dispersion and rheology, improve impact strength and mechanical properties, promote adhesion, and inhibit corrosion.
Among the early patents disclosing and claiming titanate coupling agents for use with fillers in the preparation of thermosetting resins are U.S. Pat. Nos. 4,069,192, 4,080,353, and 4,087,402. A number of patents, assigned to Kenrich Petrochemicals, Inc., were issued in the name of Salvatore J. Monte and Gerald Sugarman between 1978 and 1987, covering a variety of titanate coupling agents and resin systems. Other patents utilizing titanate additives for resin compositions to improve coupling or bonding include U.S. Pat. No. 4,163,004 for dental filling compounds; U. S. Patent 4,269,756 for glass fiber reinforced resins; U.S. Pat. No. 4,308,298 for marble dust filled resins; U.S. Pat. No. 4,363,887 for titanium dioxide filled coatings; and U.S. Pat. No. 4,397,751 for ferric oxide filled recording materials.
Other patents have disclosed the use of titanate coupling agents in unfilled resin systems for other purposes, such as U.S. Pat. No. 4,424,294 to improve flow properties in an unfilled copolyimideamide, German Patent 2,758,112 as a catalyst in a polyol system, and Japanese Patent 16,9054 to improve coating adhesion.
The prior art, however, does not suggest usefulness of such titanate coupling agents for reducing or eliminating voids in filled or unfilled resins used for potting structures such as honeycomb core and electronic components or assemblies.
Honeycomb core panels in which a hexagonal pattern (or "honeycomb") foil core is bonded between face sheets have come into widespread use, particularly in aerospace applications due to their excellent strength-to-weight ratio. Typically, aluminum or titanium honeycomb core is brazed or diffusion bonded to aluminum or titanium face sheets.
While the overall panels have considerable strength and stiffness, local areas may fail if subjected to impact or other localized high stress. For example, it is difficult to fasten other structures to such a panel without causing localized deformation or crushing of the panel. To avoid this problem, the local areas are generally potted or filled with a resin precursor which is cured in place to reinforce the potted areas and provide a higher strength attachment point. In order to save weight, which is of critical importance in many aerospace applications, only a minimum area is potted. The potted region, therefore, must have uniform high strength. Avoidance of voids or bubbles in the potted area which can severely reduce strength is of great importance. Large and complex panels must be rejected for use if excessive voids are found in potted areas. Rework of these areas is very difficult or sometimes impossible so that excessive voids can result in scrapping expensive panels. Similarly, where electronic components or assemblies are to be potted in resin, excessive voids may allow entry of moisture or may unacceptably weaken the assembly where it is to be used in a hazardous environment.
Many existing thermosetting resins, such as epoxies, have excellent strength properties and bond well to the honeycomb core material or electronic components. Despite careful processing, mixing and pouring, often excessive voids form which are not detectable until after a panel or potted electronic assembly is complete and tested, such as by ultrasonic examination.
Therefore, there is a continuing need for methods of preparing potting compositions and of performing the potting operation which will substantially eliminate voids in the potted product.